Two Roman Catholic bishops who led a central Pennsylvania diocese helped cover up the sexual abuse of hundreds of children by over 50 priests or religious leaders over a 40-year period, according to a grand jury report issued Tuesday.

The 147-page report on sexual abuse in the Altoona-Johnstown Diocese was based partly on evidence from a secret diocesan archive uncovered through a search warrant executed in August, said Pennsylvania Attorney General Kathleen Kane, who announced the findings.

"These predators desecrated a sacred trust and preyed upon their victims in the very places where they should have felt most safe," Kane said in a statement. "Just as troubling is the cover-up perpetrated by clergy leaders that allowed this abuse to continue for decades."

No criminal charges are being filed in the case because some abusers have died, the statute of limitations has expired and, in some cases, victims are too traumatized to testify, she said.

The report is especially critical of Bishops James Hogan and Joseph Adamec. Hogan, who headed the diocese from 1966 to 1986, died in 2005. Adamec, who succeeded him, retired in 2011.

Adamec cited possible self-incrimination in refusing to testify before the grand jury in November. But in a court filing, his attorney said the accusations against the retired 80-year-old bishop are unfounded. Adamec required 14 priests accused under his watch to undergo psychiatric evaluation, the filing said. Nine of them were suspended or removed from ministry, and the five who were reinstated never re-offended, his attorney wrote.

"Bishop Adamec's handling of abuse allegations has no similarity to other clergy abuse scandals," his attorney wrote.

The current bishop, Mark Bartchak, is not accused of any wrongdoing. He recently suspended a handful of priests named as alleged abusers in the report, though the grand jury said it remains "concerned the purge of predators is taking too long."

The clergy sex abuse crisis erupted in 2002, when The Boston Globe persuaded a judge to unseal files from the Boston Archdiocese in the case of a pedophile priest who had been transferred by bishops from parish to parish without warning parents or civil authorities. The scandal then spread nationwide and beyond, as Catholics and others demanded to know the full scope of wrongdoing by abusers and the bishops who kept them working with children.

Since then, dioceses across the country have been forced to release thousands of internal files on accused priests.

The Altoona-Johnstown report said Hogan covered up abuse allegations by transferring offending priests, including by sending one accused clergyman to a school for boys.

The church's own records show that priest "would have been prosecuted and convicted except that the bishop intervened and he was sent to Michigan for treatment and then placed in another parish upon his return," the grand jury found.

One diocesan official under Hogan, Monsignor Philip Saylor, told the grand jury that church officials held such sway in the eight-county diocese that "the police and civil authorities would often defer to the diocese" when priests were accused of abuse, the report said. Saylor told the grand jury that the mayors of Altoona and Johnstown even consulted him on their choices for police chief in the 1980s.

"Politicians of Blair County were afraid of Monsignor Saylor, and he apparently persuaded the mayor to appoint me as the chief of police," Altoona's former police chief Peter Starr testified.

The report contends Adamec or his staff threatened some alleged abuse victims with excommunication, and generally worked harder to hide or settle allegations of abuse than to sanction the priests accused of committing them.

The report said the bishop created a "pay-out chart" to help guide how much victims would receive from the church. Victims fondled over their clothes were to be paid $10,000 to $25,000; fondled under their clothes or subjected to masturbation, $15,000 to $40,000; subjected to forced oral sex, $25,000 to $75,000; subjected to forced sodomy or intercourse, $50,000 to $175,000.

The U.S. Conference of Catholic Bishops estimates that American dioceses have paid nearly $4 billion since 1950 to settle claims with victims.

The investigation began when Kane's office was asked to review the handling of abuse allegations at Bishop McCort Catholic High School against an athletic trainer, Franciscan Brother Stephen Baker, who worked there from 1992 to 2001. Baker killed himself in 2013 after abuse settlements with an Ohio diocese where he formerly worked were publicized. That investigation continues.